SYRACUSE, N.Y. - Motorists have gritted their teeth for decades over an incomplete highway exchange in Syracuse that can force them to drive onto city streets to get from Interstate 81 to Interstate 690, and vice-versa.

But relief is on the way: The massive billion-dollar project to replace the elevated Interstate 81 viaduct includes plans to complete the missing link.

Currently, drivers wanting to travel from the New York State Fairgrounds in Geddes to the Syracuse Hancock International Airport use I-690 east, detour about one mile through stoplights in the old Oil City area, then head north on I-81. The same is true for drivers making a return trip from airport to fairgrounds.

"We call it the missing link feature because it doesn't exist today," said Mark Frechette, the director overseeing the I-81 project for the state's Department of Transportation. "Most interstates don't have that missing link."

Filling those gaps is a major goal for state and federal transportation officials as they determine I-81's future in Syracuse. In fact, parts of the plan stretch well beyond replacing the 1.4-mile of aging viaduct near University Hill. The state's plans under consideration include reworking ramps onto I-81 from Onondaga Lake Parkway and Old Liverpool Road, and reconfiguring the West Street interchange with I-690.

All six recommended options from the state - three for a new viaduct and three for a street-level option - include that comprehensive goal of reworking of Syracuse's major crossroads. Those additional goals also will mean additional costs, construction and, most likely, land acquisitions, which also have been factored into the state's planning thus far.

Frechette said he's not sure how the two interstates came to have two exchanges instead of four. Both roadways were built in the middle of the 20th century, plopped into a century-old city.

"I can only imagine the difficulty with putting all the interchanges there," he said Wednesday.

DOT officials are gathering new data involving I-81 traffic, but could not provide Wednesday statistics on how many cars travel per day on Hiawatha Boulevard to get between the two highways.

It's too soon to know which exact lots or buildings would be needed for the I-81 solution or for the fixes to I-690 and other areas, he said. So far, the state's proposed solutions include different ideas for improving the interchange.

Today's I-81 forum at the Oncenter will launch a public comment period on the state's latest set of recommendations. The forum is from 3 p.m. to 8 p.m. Formal presentations will be made at 4 p.m. and repeated at 6 p.m. People are invited to critique the plans and share their own ideas.

Some are already questioning whether completing the exchange at I-690 should be part of the overall project - especially if it drives the cost up.

"Do we really need the expense and community disruption for these enhancements of interstate connectivity?" asked Peter Sarver, a member of Moving People Transportation Coalition, a group concerned about the future of I-81 and transit.

Frechette says yes. Transportation officials are launching the I-81 project to address the viaduct. But the project also means to improve safety at the interchange that exists now with I-690, where accident rates are four times the state average. Plus, Frechette said, some of the I-690 bridges are as old as the ones along the I-81 viaduct.

"If you're addressing the 81 bridges, you're addressing the 690 bridges," Frechette said of the interchange.

So far, the state has focused its attention on I-690 between West Street and Teall Avenue, Frechette said. But a new idea from local developer Bob Doucette and others could expand that vision.

Doucette and a growing group of developers and architects would like the state to consider using I-690 as part of the new I-81. Under that plan, motorists could drive south on the existing I-81 past Destiny USA, then use I-690 east (renamed I-81) to go out to Interstate 481 before heading on south. The idea would allow for continued traffic by the mall and businesses in Syracuse's northern suburbs and the viaduct to come down.

"That was an interesting one," Frechette said at a meeting Tuesday night about the idea. "We're going to have to do some work to figure out whether that's viable. That's certainly something we need to look at."